The present invention generally relates to apparatus and method for compressing a fluid (e.g., a gas such as air) at high temperature (i.e., greater than or equal to about 350° F.) and high pressure (i.e., greater than or equal to about 2000 pounds per square inch). More specifically, the present invention relates to apparatus and methods relating to piston rings for use in compressors operated to produce high pressure fluids at high temperatures.
Air compressors used to produce high pressures in excess of 2000 psi, which may exceed 5000 psi, at high temperatures in excess of 350° F. may be utilized in, for example air-recharge systems on an aircraft. Air-recharge systems may be used to provide emergency power to the aircraft. Accordingly, air compressors used in these systems may need to be reliable, and may need to have a relatively long operational life time in excess of 10 years of service. However, the extreme temperatures and pressures of such service may not be conducive to such long operational lifetimes.
The basic operational principle of such fluid reciprocating pumps or air compressors is to draw fluid into a cylinder through a one-way valve by linear movement of a piston sliding in sealed relationship within the cylinder and then to expel the fluid from the cylinder through a further one-way valve by reversing the direction of movement of the piston within the cylinder. The cylinder may be defined by a cylinder wall, the piston being reciprocally mounted within the cylinder, a crankshaft being coupled to the piston, and arranged such that rotational movement of crankshaft causes reciprocating axial movement of the piston within the cylinder. The piston disposed in the cylinder may thus define an annular cavity between the piston and the cylinder wall.
A compressor useful to compress a fluid (e.g., a gas such as air) to pressures in excess of 2000 psi at temperatures in excess of 350° F. may comprise a plurality of cylinders. The cylinders may be arranged in serial communication, with the first in the series receiving the lowest pressure intake (e.g., atmospheric pressure), and exhausting to the intake of another cylinder in the series. Accordingly, each cylinder may increase the pressure and the temperature of the fluid until the final stage wherein the high pressure, high temperature fluid is discharged from the compressor, for example into a storage vessel.
Because the diameters of the piston and the receiving cylinder bore differ, a sealing arrangement is needed. One approach to sealing the annular cavity at the piston/cylinder interface is to size the piston to be fit in the cylinder such that the cavity is sealed with a film of lubricant such as an oil film. This approach may use oil from a sump, which enters the cylinder on the side of the piston away from the fluid being pumped or compressed. Generally, an oil scraper is provided to recirculate oil spread up the wall of the cylinder back to the sump via the interior of the piston. However, sealing at temperatures above 350° F. and pressures in excess of 2000 psi may become difficult due to thermal expansions of the cylinder wall and piston, along with viscosity breakdown of the lubricating oil.
An approach to sealing the piston/cylinder interface that does not rely on an oil film is to provide the piston with one or more circumferential grooves within an upper end of the piston. Piston rings are installed in these grooves, which rings have a slightly larger outer diameter than the piston. The piston rings generally bear directly against the cylinder wall and create a seal between the sides of the piston ring groove and the cylinder wall. The ends of the piston ring may be tapered and/or fitted to interlock, such that when the piston ring is installed on a piston which is sealingly fit within a cylinder, essentially no gap is present between the piston ring ends.
The degree to which the piston ring and the cylinder wall of a compressor contact one another can vary significantly due to disparate thermal expansions, thermal deformations, inlet and outlet pressures, and the like. Such forces may have a limiting effect on the usefulness of a piston ring and/or a cylinder. Under temperatures in excess of about 350° F. and final pressures over about 2000 psi, especially in the absence of external lubrication, a piston ring may wear to a point such that the compressor is unable to achieve a high pressure. Such high pressure and high temperature may also cause the cylinder walls to become scored, thus limiting the usefulness of the compressor. Accordingly, compressors operated to deliver relatively high outlet pressures (e.g., pressures in excess of 2000 psi) at relatively high temperatures (e.g., temperatures exceeding about 350° F.), may demonstrate piston rings that wear quickly, or cylinder bores which become scored by, for example, dry running of piston rings (i.e., without an external lubricant) therein. Thus, operation of compressors at relatively high outlet pressures and at relatively high temperatures may substantially limit the useful lifetime of a piston ring, and/or of the compressor itself.
As can be seen, there is a need for a piston ring that will prevent premature wear, and/or scoring of the cylinder bore, and which can provide for compression of a fluid over 2000 psi at temperatures greater than or equal to about 350° F.